1. Field of Invention
This invention concerns an optical apparatus. In particular, it concerns an optical apparatus used in projection equipment.
2. Description of Related Art
Projection equipment is generally either the type that uses liquid crystal (LCD) panels or the type that uses DLP.
The type that uses liquid crystal panels has both a one-layer variety and a three-layer variety, but with either variety, the beam emitted from the light source is split into three colors (RGB—R=red, G=green, B=blue), and after transmission coordination of the light corresponding to image information in the liquid crystal panel, the three colors that pass through the panels are combined and projected onto a screen.
In the type that uses DLP, on the other hand, the beam emitted from the light source passes through a rotating filter by which the RGB domains are divided and formed, irradiated by means of time division such as a space modulation element (what is called a light modulation device, or specifically a DMD element), and specified beams are reflected by the DMD element and projected onto the screen. A DMD element is spread with several million small mirrors, one per pixel, and by controlling the direction of each small mirror it is possible to control the projection of light.
In comparison with the LCD type, the DLP type has a simpler optical system and there is no need to use as many as three layers of LCD panel, and so the equipment as a whole has the advantage of being smaller and simpler.
On the other hand, high pressure discharge lamps with high mercury vapor pressure are used as light sources for the projection equipment. The reason for the high mercury vapor pressure is to enable a high light output in the visible spectrum.
Furthermore, these discharge lamps (simply called “lamps” hereafter) are inserted in concave reflecting mirrors in the shape of an ellipsoid of revolution (roughly rice bowl shaped) in order to brighten the image projected on the screen. Using a concave reflecting mirror can efficiently concentrate the light radiated from the lamp on a screen of limited area.
In recent years it has become more common for projection equipment, particularly that used for presentations, to be used away from one's main location, and so there have been strong demands for smaller and lighter equipment in the sense that it can be carried more easily.
When smaller size is demanded of projection equipment, reduction of the optical apparatus (discharge lamp and concave reflecting mirror) incorporated in the projection equipment is naturally demanded as well.
It is also natural to say that, given such constraints on size and shape, it is necessary to heighten the efficiency of use of the light radiated by the lamp.
FIG. 10 shows the structure of a reflecting mirror devised to heighten the efficiency of use of light.
The reflecting mirror 200 is constituted of an elliptical reflecting mirror portion 210 and a spherical reflecting mirror portion 220, respectively located to the front and rear in the direction of radiation. Specifically, the elliptical reflecting mirror portion 210 is formed toward the front opening of the reflecting mirror 200, and the spherical reflecting mirror portion 220 is formed toward the rear opening or neck.
With this constitution, light beam L1 radiated by the lamp 100 toward the neck of the reflecting mirror is reflected by the spherical reflecting mirror portion 220 and returns in the direction of the arc (light beam L2), after which it passes through the discharge arc toward the elliptical reflecting mirror portion 210 (light beam L3) and is reflected toward the front opening (light beam L4).
Compared with the use of a reflecting mirror with an elliptical surface only, this constitution is certainly able to improve the efficiency of use of light in order to be able to use the light that is radiated or reflected in the vicinity of the neck of the reflecting mirror.
In the event that the volume of the electrode is enlarged, however, there is still the problem that the light radiated by the discharge arc (L1) is blocked by the body of the electrode, or the light reflected by the reflecting mirror 200 is blocked by the electrodes or other constituent parts of the lamp.
The structure shown in FIG. 10 is described in JP-A-H3-266284 of 1991 and JP-U-S63-162320 of 1988, for example.
Patent Document 1 JP-A-H3-266284 of 1991
Patent Document 2 JP-U-S63-162320 of 1988
Patent Document 3 JP-A-2002-298625 (corresponding to US-A-2004/0070735)